Friday night's air strikes on targets near Baghdad were cleared by Tony Blair and George Bush after requests from military commanders increasingly worried by the threat to their aircraft posed by new Iraqi radar and missile systems, defence sources said yesterday.

The RAF and the US air force were concerned at what they saw as heightened risks for their pilots. Whether there is a legal basis for the no-fly zones from which the US and Britain have barred Iraqi planes since the Gulf war ended in 1991 was not an issue for them, but is a matter of controversy for the politicians.

British ministers argue that the zones are being maintained for humanitarian reasons - to reduce the ability of the Iraqi regime to attack its opponents among the Shia in the south and the Kurds in the north.

The air forces sought political clearance to act after registering serious threats from Iraq's new integrated air defence system, coordinated by fibre optic communications and bolstered by a growing number of SA-6 surface-to-air missiles supplied - according to intelligence sources - by Serbia and Ukraine.

British Tornado jets cannot fly as high as US F-15 aircraft, nor are their defence systems as good. That no Tornado had been shot down, British defence sources said yesterday, was a combination of "skill and luck".

Requests to change the rules of engagement to allow British and US aircraft to bomb military targets whether or not they were inside the no-fly zone were made early this year by British and American commanders frustrated at restrictions imposed by the Clinton administration.

The military's argument is that Iraqi installations exercising command and control for the zones can be located beyond the zone boundaries which were set by Britain and the US at the suggestion of the former prime minister John Major.

President Bush is believed to have discussed these requests at a special White House meeting on Iraq earlier this month. They were almost certainly discussed at a meeting two weeks ago in Munich between Donald Rumsfeld, the defence secretary, and Geoff Hoon, his British counterpart, and taken up by Robin Cook in Washington last week.

But defence sources insist that the timing of the raids was a military decision. Eight RAF aircraft were involved and were given just one target, in the southern no-fly zone.

The RAF is not equipped with US "stand-off" weapons which enable pilots to fire their weapons farther from the target.

British Paveway laser-targeted bombs were used in Friday night's attack, which involved four Tornado bombers, two Tornado defence support aircraft, and two VC-10 tankers.

British defence sources expressed surprise at the response to the raids, suggesting that it had been hyped up by the media with shots of the Baghdad skyline lit up by anti-aircraft fire.

Their reaction perhaps can be explained by the fact that this is generally a "forgotten war", though British aircraft have dropped well over 100 bombs on ground installations where Iraqi targeting equipment has locked on to the western patrol aircraft. The 10-year US-British operation has cost taxpayers more than £900m.

Contrary to claims yesterday by the government - including Brian Wilson, the new Foreign Office minister responsible for Iraq - the no-fly zones are not sanctioned by any UN security council resolution.

The British and US governments justify them by referring to security council resolution 688 of April 1991, which condemned "the repression of the Iraqi civilian population in many parts of Iraq [and] demands that Iraq ... immediately end this repression".

Few now believe the patrols serve any purpose in the south, the reason France gave in 1998 for stopping its air patrols.

Mr Hoon said at the weekend that in international law Britain was "perfectly entitled to deter aggression".

The Commons foreign affairs committee said last year that "at the very least, the doctrine of humanitarian intervention has a tenuous basis in current customary law", while the defence committee said: "The precise legal basis for the no-fly zones is controversial."

Mr Cook said yesterday that President Saddam had not given up his claim to Kuwait or wish to get weapons of mass destruction.

But the no-fly zones were not imposed to deter these threats.

Turkey, from where US and British aircraft patrol the northern zone but which also conducts attacks on Kurds, was one of many countries to criticise the latest raids.